For the first time in a generation, public schools have won broad freedom to teach boys and girls separately, stirring a new debate about equality in the classroom.
The Education Department announced rules that would make it easier to create single-sex classes or schools, a plan that's been expected for almost three years.
The move comes as the value of same-sex education is in doubt.
Research shows mixed results, as even the department's own review says. Yet Education Secretary Margaret Spellings said more parents deserved to have the option.
About 240 public schools offer same-sex education in the United States, up from just three in 1995, according to the National Association for Single Sex Public Education.
At one inner-city public high school in Chicago, girls raise their hands high in science lab and there are no boys.
"You don't have to worry about dumbing yourself down as some people say," said sophomore Jasmine Perry. "Girls do so they won't intimidate guys especially in math and science."
It's a luxury that has been traditionally reserved for daughters of the upper class.
"Wealthy children across this country have had these options forever. Poor children haven't," said Arne Duncan, CEO of Chicago Public Schools. "We're trying to dramatically change that."
The Chicago Schools chief plans to open 20 more all-girls and -boys schools. It's an attempt to help fix the city's ailing public school system where graduation rates have hovered around 68 percent.
In the past, anti-discrimination laws have made it all but impossible for public school districts to offer single-sex classes.
Now, federal regulations have eased and school districts around the country are looking at this model as a way to improve education.
Many organizations are against the idea.
"Dividing men from women and boys from girls when they will spend their lives living and working together is a terrible idea for public education," said Kim Gandy, president of the National Organization for Women.
But educators at one Chicago girls school say that is not what they're seeing.
Last year every single senior graduated.